1. Field of the Invention
The present invention relates to a driver circuit for electronic components, with an amplifier and a control current transistor, which generate a control signal from a reference signal, with a driver current mirror, and with a control current switch, which is located between the amplifier and the driver current mirror and which selectably isolates the driver current mirror from the control current transistor or connects the driver current mirror to the driver current transistor, wherein the driver current mirror, when in a state connected to the control current transistor, amplifies the control signal into a driver signal.
2. Description of the Background Art
Driver circuits are used, for example, for driving laser diodes. In simple conventional driver circuits, the control current is stabilized by a negative feedback in which a feedback signal input of an amplifier is supplied through feedback with a feedback signal coupled out of a control branch of the driver current mirror.
In driving an electronic component, a driver current serving as a driver signal should be settable by a reference current with which the amplifier is modulated. In this context, the gain of the amplifier circuit, in other words the quotient of the driver current and the reference current, and also the value of the reference current, should be as independent as possible of the value of the driver current and the temperature of the components involved, and should also be as consistent as possible when components to be driven and driver circuits are produced in large quantities.
The gain of the amplification for laser diode drivers for current CD/DVD drives is approximately 1 MHz. On account of this relatively low bandwidth, the driver current, which must be switched in the sub-nanosecond range, cannot be accomplished through corresponding pulse control of the reference current. Rather, the keying is accomplished by enable signals which, with the aid of the aforementioned control signal switch, selectably feed the control current immediately ahead of the driver current mirror serving as an output stage into either the driver current mirror or into other circuit components.
The desired current conversion ratio between the driver current and the control current of the control current mirror is typically in the range between 5 and 20. A high conversion ratio is desirable for reasons of low current consumption. A rather lower conversion ratio is preferred from the standpoint of the fastest possible switching speed.
Current mirrors are implemented using transistors. It is known that the current gain beta of bipolar transistors, which is the quotient of the collector current IC and base current IB, is subject to relatively large dispersion (for example, between 30 and 100) in the large-quantity production of integrated circuits. Moreover, the current gain is dependent on the collector current. The aforementioned dispersions carry through to the characteristics of the driver current mirror, so that its gain, which is defined here to mean the quotient of the driver current and control current, has dispersion, with the result that the overall gain of the driver circuit likewise exhibits the undesirable dispersions.
A conventional driver circuit, which is illustrated in FIG. 6, compensates for dispersions using a compensating circuit that has a current mirror, which is hereinafter referred to as the compensating current mirror to distinguish it from the driver current mirror. The compensating current mirror is always connected to the amplifier. The feedback that stabilizes the amplifier is connected to a control branch of the compensating current mirror. This distinguishes the aforementioned known driver circuit with compensating current mirror from the likewise known simple driver circuit whose feedback is supplied from the control branch of the driver current mirror.
Compensation, according to the conventional art, is based on the fact that the compensating current mirror coupled to the feedback of the amplifier is largely as identical as possible to the driver current mirror, and in particular has the same effective current gain. As is discussed in greater detail below, the gain of the overall arrangement can then be set by dimensioning certain resistances. The matching of their characteristics is inherently good in a monolithic integration of the circuit, in contrast to the characteristics of the transistors.
However, the conventional art compensation requires a comparatively large amount of current, it produces an additional noise contribution, and the current mirror error of the compensating current mirror caused by production tolerances enters directly into the amount of the gain. It is critical in particular that increasingly stringent requirements for low noise levels in the driver current cannot be met with the prior art compensating circuit.